The present invention relates to a method and apparatus for reducing the amount of fuel required to perform work in a vapor cycle by eliminating unnecessary phase changes. In particular, the present invention relates to a limited expansion vapor conversion cycle that is a modification of the conventional Rankine steam cycle.
In the known Rankine cycle, fuel is burned to heat a boiler vessel or vapor generator containing water, thereby generating steam. The steam is then passed through piping to a turbine or piston device to produce work. The pressure of the steam at the outlet end of the turbine is reduced substantially over the pressure at the inlet end of the turbine. At the outlet end of the turbine the depressurized steam is conveyed to a condenser where the steam is cooled and a partial vacuum is created, thereby producing a phase change from steam to water. The water is then returned to the boiler vessel or vapor generator, and the cycle is repeated.
According to the prior art, the pressure of the steam at the outlet end of the turbine is reduced to the maximum extent possible in comparison with the steam pressure at the inlet end of the turbine. Thus in the typical Rankine cycle two phase changes are used. First, water is heated to form steam before entering the turbine, and second, steam is condensed to form water after flowing through the turbine. For each pound of steam, approximately 970 BTU's of heat are required in the phase change from water to steam.
It is an object of the present invention in a vapor cycle for work to minimize or eliminate the phase change from steam or other vapor to water or other condensate, thereby eliminating the need for accomplishing the reverse phase change from vapor to condensate in the boiler. In this manner, the amount of fuel used in a steam or vapor cycle apparatus is reduced.
It is known in the art that a turbine is an engine which derives work from a steady stream of fluid. The working forces of the turbine come from changes in the momentum of the fluid. The turbine contains guides or blades that are moved by the fluid, and passages between the guides that also move as the fluid passes therethrough. In the moving passages of the turbine, the velocity of the stream of fluid relative to the moving guides is reversed in direction. The change in direction and velocity results in a force on the moving guides which can do work.
The turbine nozzle is a passage which is designed to increase the velocity of a flowing stream. The ratio of the velocity of the stream to its specific volume is referred to as the mass rate of flow per unit area.
The maximum flow or work through a turbine exists when the exhaust chamber or outlet pressure divided by the initial or inlet pressure produces a ratio of a little greater than one half. The pressure ratio at this section of maximum flow or work per unit area is referred to as the critical-pressure ratio. The critical-pressure ratio is applicable to both converging and converging-diverging nozzles in the turbines.
According to the present invention, there is provided a limited expansion vapor system whereby the phase change from steam or other vapor to water or other condensate is substantially reduced or eliminated. This is accomplished by utilizing a compressor or condenser that restricts the pressure of the vapor at the outlet end of a work deriving means to approximately one-half the inlet pressure, thereby restricting the expansion of the vapor to a minimum.
If a compressor is used, both condensate and compressed vapor may be produced. The compressed vapor is returned to a boiler means to be reheated, and the condensate is pumped to the boiler means. If a condenser is used, the output is condensate, which is pumped to the boiler means for reheating and a phase change to vapor. The embodiments described herein relate to a steam cycle apparatus utilizing water, but it is contemplated that other fluids which undergo a reversible phase change from liquid to vapor can be used.
Thus, one embodiment of the present invention comprises a boiler means for heating a mixture of vapor and liquid to produce a phase change to vapor; means for deriving work from the vapor produced by the boiler means, said work deriving means having at least one stage; means for compressing vapor output from the work deriving means; means for cooling said vapor output during compression; means for returning compressed vapor output to the boiler means; and means for pumping condensate produced by the compressor to the boiler means. The compressor capacity is matched to the vapor output of the final stage of the work deriving means so that the pressure of the vapor output is maintained at approximately one-half the final stage vapor inlet pressure, thereby restricting the expansion of the vapor output, and the compressor work. Inter- or intrastage cooling further reduces the compressor work.
According to an alternative embodiment there is provided a boiler means for heating a liquid to produce a phase change to vapor; means for deriving work from the vapor produced by the boiler means, said work deriving means having at least one stage; means for condensing the vapor output from the work deriving means to produce condensate; and means for pumping the hot condensate to said boiler means. The condenser capacity is chosen so that the pressure of the vapor output is approximately one-half the vapor inlet pressure of the final stage, thereby restricting the expansion of the vapor output, reducing condenser requirements, and reducing the need for feed water heating.
The invention also includes methods of converting heat to work by operating the above-identified embodiments of the vapor cycle apparatus of the present invention. Other advantages, objects and features of the present invention will become apparent upon reading the following detailed description of the preferred embodiment in conjunction with the accompanying drawings, which are schematic representations of the present invention.